1. Field of the Invention
This invention relates to interbody bone fusion devices, and more particularly to an apparatus and method for the delivery of electrical current to a spinal fusion implant and to interbody fusion material for inducing bone growth and aiding in spinal arthrodesis.
2. Description of the Related Art
The spine may be fused along any of its various surfaces, or internally within the interspaces of the vertebrae. Various interbody fusion devices have been developed to promote interbody fusions of the spine, such as that of Michelson, U.S. Pat. No. 5,015,247, issued on May 14, 1991, Brantigan U.S. Pat. No. 4,743,256, issued on May 10, 1988, and others. Such devices have helped to achieve spinal fusion by providing structural support, presenting bone promoting substances to the fusion site, increasing the surface area available to participate in the fusion, and by being both self-stabilizing and stabilizing to a spinal segment.
During normal bone repair, the area around the fracture of the bone exhibits negative charge. The application of electrical current to negatively charge a site in which spinal fusion is desired simulates the bone's own normal repair process and promotes osteogenesis. The application of electrical current to negatively charge a site in which osteogenesis is desired, creates an electrochemical reaction (4e-+O.sub.2 +2H.sub.2 O.fwdarw.4OH.sup.-) which lowers the oxygen tension (decreasing the O.sub.2) to stimulate osteoblastic activity and promotes bone formation. Further, the formation of the hydroxyl radical (OH.sup.-) raises the local tissue pH which of itself is favorable to bone production and further promotes increases in the presence of alkaline phosphatase, a very potent stimulant of bone formation in it's own right. Still further, there appears to be a direct effect of electrical current to present a negative charge at the cellular level so as to upset the resting electrical potential of the cell membrane with a resultant electrical perturbation within the cell, the net effect of which is promotional to the cellular activity of bone formation. Finally, the electromagnetic field generated by the passage of electrical current appears to be independent of that current (on the basis of magnetism alone) to be promotional of bone growth, though the mechanism remains unknown.
Conversely, the application of electrical current to positively charge an area of bone inhibits osteogenesis and thus inhibits bone formation. Therefore, the application of electrical current to deliver positive charge to an area of bone may be used to control the bone fusion process so that it does not occur in undesired areas such as within the spinal canal.
The bone fusion process is a race against time, for eventually, the body will give up its attempt to complete that process. Well-known within the field of surgery is the use of electrical current delivered internally, or applied externally relative to a patient's body to promote bone growth and thus promote the bone healing or fusion process. However in regard to the spine, none of the interbody fusion devices of the past incorporate the use of electric current to stimulate bone growth, to increase the rate of osteogenesis and the spinal fusion process.
To date the use of electric current to promote bone growth in the spinal fusion process has taken two forms. The first is the use of an internally implanted electrical pulse generator, with a cathode wire leading from the pulse generator being wrapped about a bone plug harvested from the patient's body which is then inserted into the intervertebral space. These devices however have been continually plagued with problems that include breakage of the lead wires from the generator to the fusion site and a second surgery to remove the generator implanted in the patient's body at a remote location to the fusion site after the service life of the battery has expired. The power supplies of these implantable generators have been ineffective due to their limited service life, which may be shorter than the time needed to attain solid fusion, and problematic due to the potential for tissue damage in the event of a leak. The latter concern prompts most physicians to perform a second surgical procedure to explant the generator and internal battery supply. The additional surgery to explant the device increases the risk of infection and danger to the patient, and results in unnecessary additional costs.
The second form in which electric current has been used in the past to stimulate spinal fusion required the wearing, external to the body of the patient, of an electromagnetic coil or coils. Unfortunately, neither of these methods when utilized in conjunction with the known methods of interbody arthrodesis has proven fully effective.
Therefore, a need exists for the means and method of improving upon and/or perfecting the conjoined use of an improved interbody fusion device other than bone alone, and the promotion of bone growth with electrical current.